Separate dispute

Meanwhile, South Korea is refusing to allow a Japanese warship to dock at its port during a joint naval exercise because of disputed islands, according to reports.

Tokyo has lodged a protest with Seoul over the refusal during an exercise that also involves the US and Australia, reports said, with one diplomat calling it "extremely rude".

The four-nation drill, scheduled to take place on Wednesday and Thursday, is aimed at coordinating a response to possible trafficking of weapons of mass destruction, Japanese defence officials said.

In the original scenario a Japanese vessel was to dock in the city of Busan, but South Korean authorities refused to grant it permission, Japan's broadcaster NHK said.

The Sankei newspaper reported a similar story, citing a Japanese diplomat in Seoul as saying: "It is extremely rude as a host country of a multi-nation military drill."

The US-led drill, known as the Proliferation Security Initiative, has been held most years since 2003.

Japanese ships were permitted to make a port call in the 2010 drill in South Korea, a spokesman for Japan's Maritime Self-Defence Force said.

This year's drill will focus on boarding inspections in waters between Japan and South Korea, "so we don't necessarily have to make a port call in Busan", the naval spokesman said.

The Sankei newspaper said Tokyo had considered withdrawing from the joint drill in the face of Seoul's refusal, but Washington mediated and rewrote the scenario so that the Japanese ship's port call was unnecessary.

A Japanese foreign ministry official in charge of the drill declined to comment on the reports, citing "consideration into relations with other countries".

Ties between Tokyo and Seoul went into virtual freefall in August when South Korean President Lee Myung-Bak made a surprise visit to Dokdo, a pair of islands that lie between the two countries, which Japan claims as Takeshima.

His call for an apology from Japan's revered emperor, for crimes committed by forces who occupied the Korean peninsula for much of the first half of the 20th Century, was rounded on by Japanese leaders.

Diplomatic exchanges were stymied and a war of words erupted.

Tokyo's relationship with its former colony is often tense, despite their close economic ties, with historical grievances informing exchanges.